In the second millennium, electronic devices are ubiquitous, and it is certain that the number, variety and sophistication will continue to proliferate. Many of these universally available electronic devices employ radio frequency (RF) signals, including radios, televisions, cellular phones, computers, etc. In addition, more and more electronic devices are now activated by remote controls or wireless modems that transmit and receive RF signals, for example, automobiles, garage doors, cordless phones, fireplaces, toasters, microwave ovens, etc.
Consequently, there exist a multiplicity of antennas that are used to transmit and receive the various RF signals. Some antennas are designed to maximize transmission over distance (such as for satellite or airplane communication), others are designed to be low-profile for high speed and high turbulence applications (such as for airplanes or ships), while others are designed to be as small and compact as possible (such as for remote control devices or RFID tags).
Typically, these antennas are intended to transmit and receive signals having frequencies within a defined range, and the dimensions and geometry of a particular antenna limit its usefulness to a relatively narrow band of frequencies. For certain applications, however, it may be desirable to be able to monitor a wider band of frequencies. In many commercial and government applications, for example, there is a need to communicate via many different radios operating at several bands of interest. Antennas in common vehicular applications now cover cellular phones operating at 1000, 1800 and 2500 MHz; radios in VHF and UHF bands operating at 20-500 MHz; other entertainment bands, such as TV, operating at 100-600 MHz; and garage door openers operating at ˜200-400 MHz. In addition to the above, government vehicles may have requirements to communicate via a range of secure RF bands in very wide frequency range, for example from 20 MHz-10 GHz. The antenna system of the present invention provides coverage over this entire frequency range.
Broadband antennas, or those capable at operating at more than one range of frequencies, are well known, but typically have less desirable gain characteristics than narrow-band antennas. For applications requiring acceptable gain at a variety of frequency bands, multiple-antenna devices have been developed. A drawback to the multiple-antenna approach, however, is that such a device takes up more space at its point of attachment and may be more complicated and fragile than single antenna designs. This may not be acceptable, for example, in mobile applications. An advantage of present invention is that it is packaged as a single antenna, and as such is compact, robust and has a small footprint, allowing it to be easily attached to a wide range of substrates, including vehicles.
Other approaches to broadband antenna design include using a single broadband antenna such as a biconical that extends the entire frequency band, or using a frequency independent antenna such a spiral. A problem with both of these approaches is that as the frequency range expands, the antenna dimensions become increasingly large in diameter. For certain applications, an excessively large diameter antenna is impractical or even impossible. A novel feature of the present invention is the tubular shape of the antenna system, having a relatively small diameter that allows packaging of the antenna for a variety of applications, including vehicular applications.
Yet another approach to providing a broadband antenna is to use a frequency tunable antenna. A tunable antenna requires information regarding the frequency band of interest in order to tune the antenna to the desired frequency. This becomes a major handicap for tunable antennas, however, when the frequency of operation of the system is not known. An example of such systems is the “frequency hopping” radio communications system, where the frequency of operation is changed to reduce interference from unwanted sources. The “frequency plan” for hopping is not always known ahead of time, which can hinder the ability of a frequency tunable antenna in receive mode to be used in hopping systems. In general, it is inconvenient and unreliable to make manual adjustments every time a frequency change is needed. Instead of manual tuning, a tunable antenna may have electrical tuning capability. A drawback of such a tunable antenna, however, is the complexity and cost of active components that are required for the adjustable tuning. The present invention overcomes all such drawbacks of tunable antennas, as it comprises a single passive structure with no active components.
An additional feature that is desirable for vehicular antenna applications is having an omni-directional capability, i.e., having a radiation pattern with adequate gain over 360 degrees of coverage in the azimuthal plane and at low elevation angles near horizon, such as when the antenna is mounted vertically on a vehicle. Vehicles on the move may change orientation rapidly, and thus it is preferable that a vehicular antenna be able to maintain communication without adjustment. The antenna system of the present invention provides such omni-directional capability, and does so over a wide bandwidth.
Another advantageous feature of the present invention is having broadband impedance characteristics that allow the antenna system to operate with common RF systems (radios). Typical voltage standing wave ratio (VSWR) of the antenna of the present invention is less than 3:1 over the 500:1 frequency span. This allows the antenna to operate in both transmit and receive modes with a relatively small degradation in performance.
Antennas that utilize dipoles, biconical structures and monopoles to achieve enhanced bandwidth are known in the art. For example, U.S. Pat. No. 4,496,953 to Spinks, Jr. et al. discloses a dipole antenna, that, like the present invention, uses couplers to couple energy from one radiator to another. In the Spinks, Jr. et al. antenna, however, energy is coupled between the two arms of the dipole, whereas in the present invention, coupling takes place in the low band to create a monopole and it is then isolated from the monopole to create an asymmetric dipole that covers the mid-band. Further, Spinks, Jr. et al. disclose a bandwidth of only approximately 2:1, much narrower than that of the present invention.
U.S. Pat. No. 4,835,542 to Sikina, Jr. discloses a biconical antenna claiming a 10:1 bandwidth, which, compared with the present invention, is only a moderately broadband antenna. The size of the Sikina, Jr. biconical antenna is determined by the lower extent of the frequency of operation, resulting in a biconical diameter that is rather large, compared to that of the present invention.
U.S. Pat. No. 5,257,032 to Diamond et al. discloses a broadband antenna system including a spiral antenna and dipole or monopole antenna. Dipole arms are added to improve the bandwidth of the broadband antenna, while a dipole or monopole antenna are added to improve performance at low frequencies. In contrast, the present invention employs an asymmetric dipole antenna and makes additional use of that structure to excite a monopole antenna. Unlike the Diamond et al. antenna which uses a single feed for each antenna, the present invention uses two separate feeds, one for each of two component antenna structures, the monopole and the combined asymmetrical/biconical dipole. Furthermore, the present invention is designed to provide an omni-directional, vertically polarized beam. In contrast, the spiral antenna of Diamond et al. is circularly polarized, with an associated loss compared to the vertically polarized antenna of the present invention.
U.S. Pat. No. 5,892,486 to Cook et al. discloses a dipole antenna array arranged with a balun to make improvements in the bandwidth performance. Having only an approximately 1.75:1 bandwidth, this is not an ultra-broadband antenna.
U.S. Pat. No. 6,154,182 to McLean discloses a biconical antenna that is designed to have a 10:1 bandwidth. It is a wire biconical antenna to which a plate can be added or removed from the top of the antenna. Adding the plate enables performance at the low end of the band. Removing the plate improves performance at the high end of the band. This differs substantially from the present invention in that the McLean design requires manual changes to be made to the antenna to achieve the larger bandwidths. Furthermore, in order to provide extended low-end performance, the McLean-antenna become large in diameter, similar to the Sikina, Jr. antenna described above.
U.S. Pat. No. 6,239,765 to Johnson et al. discloses an asymmetric dipole antenna assembly. Unlike the present invention, this antenna is printed and is not a broadband antenna.
U.S. Pat. No. 6,667,721 to Simonds discloses an antenna consisting of a bicone with exponentially tapered reflector fins. This antenna has a 135:1 bandwidth (FIG. 4 levels below −10 dB) and like the present invention, uses a bicone antenna to match the impedance. While the Simonds antenna has a performance similar to that of the present invention, its design is significantly different in that it does not integrate any additional antennas to the bicone to improve the performance. Further, in order to achieve the disclosed low-band performance, the Simonds antenna design substantially exceeds the diameter of the present invention. Simonds discloses that “the fins function to reduce the traditional bicone antenna diameter,” yet the bicone extends in width and results in an overall width-to-height aspect ratio of approximately 1. In contrast, the present invention is designed to achieve a similar performance with an aspect ratio of only 0.1 (10%).
U.S. Pat. No. 6,693,600 to Elliot discloses a wire monocone and an additional radiator (which may be a large monopole) to provide increased bandwidth of approximately 4:1 (FIG. 8 levels below −10 dB). Unlike the present invention, which integrates a monopole and an asymmetric dipole with biconical feeds and provides two feeds, the Elliot antenna uses a single cone, integrates it with a monopole and provides a single feed point. In further contrast with the present invention, the Elliot antenna has a wide aspect ratio like that of the Sikina, Jr. and Simonds antennas.
U.S. Pat. No. 6,919,851 to Rogers et al. discloses a thin broadband monopole/dipole antenna that uses lumped circuits to increase the bandwidth of the antenna. Unlike the present invention, no combining or conical feed sections are used.
U.S. Published Pat. Application No. 2003/0034932 to Huebner et al. discloses a planar monopole above a co-planar rectangular sheet. The sheet is connected to ground and the antenna is excited using a coaxial feed. The Rogers et al. antenna may also be viewed as an asymmetrical planar dipole. This antenna has a 5:1 bandwidth, whereas the present invention has a much larger bandwidth.
As described above, antennas known in the art lack the combination of advantages found in the antenna system of the present invention. The need exists, therefore, for an antenna capable of operating over a wide range of frequencies that is compact, robust, occupies a relatively small footprint—all with a narrow aspect ratio. The present invention provides these features in a single tubular antenna structure that, because of the innovative combination of a biconical dipole element and an asymmetrical dipole element, also functions as a monopole. Incorporation of the monopole in this way substantially increases the low frequency performance without excessively increasing the length (height) of the overall antenna. The design of the present invention is thus an improvement over conventional dipole antennas capable of operating at the same frequencies. The present invention therefore provides ultra-broadband coverage, i.e., acceptable gain in the low frequencies, intermediate frequencies and high frequencies. As a result, the present invention has application where it is desirable to monitor the RF spectrum ranging from 20 MHz to 10 GHz.
Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.